1. Field of the Invention
The invention resides in the field of electrolytic devices and more particularly relates to chlor-alkali or alkali metal chloride systems utilizing a multiplicity of cells each containing fluid impermeable, permselective perfluorocarbon membranes.
2. Description of the Prior Art
The electrolysis of alkali metal chlorides in a multiplicity of cells each with a cation selective membrane for the production of chlorine and alkali hydroxides is well known, particularly with respect to the conversion of sodium chloride. In the sodium chloride process the electrolysis apparatus typically consists of several cells divided into anolyte and catholyte compartments by permselective cation membranes. Brine is fed to the anolyte compartments and water to the catholyte compartments. A voltage impressed across the cell electrodes causes the migration of sodium ions through the membranes into the cathode compartments where they combine with hydroxide ions formed from splitting of water at the cathodes to form sodium hydroxide (caustic soda). Hydrogen gas is also formed at the cathodes and chlorine and oxygen gases are formed at the anodes.
The efficiency of these cells for production of caustic and chlorine depends upon how they are operated, that is, the balancing of the chemical parameters of the cell, the process liquids and further how the cells are constructed, that is, what materials are used to form the components. Maximizing the efficiency generally results in the production of noxious liquid wastes. The cation permselective fluorocarbon membranes which separate the anode compartments from the cathode compartments are fluid impermeable and effectively prevent bulk flow of brine into the caustic in the cathode compartment, of hydrogen into the chlorine in the anode compartment and of chlorine into the hydrogen in the cathode compartment. They usually contain carboxylate, sulfonate and/or sulfonamide active groups. They suffer the disadvantages of all cation selective membranes in chlor-alkali cells, that is, while highly selective to monovalent cations (as compared to anions) they permit the passage of non-monovalent cations from the anode compartment to the cathode compartment and of some hydroxide anions from the cathode compartment to the anode compartment. The encounter of the flux of non-monovalent cations with the flux of hydroxide anions results in the precipitation within the cation selective membrane of the insoluble hydroxides of some non-monovalent cations, including for example, magnesium, calcium, strontium, barium and iron. These precipitates increase the electrical resistance of the membranes and if allowed to build up decrease the efficiency of the membrane for transport of monovalent cations and may eventually destroy the physical integrity of the membrane. It has been proposed in U.S. Pat. No. 3,793,163 to mitigate such precipitation by adding phosphate to the brine influent to the anode compartments. A few hundred parts per million of phosphate are typically used and for example result in the formation in the anode compartments of an isoluble gel precipitate containing calcium and phosphate. Such phosphate addition does appear to reduce the rate of precipitation of calcium hydroxide inside the cation selective membrane. An excess of phosphate must be used compared to the amount of calcium present and if not carefully controlled may have deleterious effects on the over potential and life of the ruthenium oxide type catalytic anodes frequently used in membrane chloralkali cells. Such phosphate also constitutes an undesirable secondary feed which must be carefully controlled and adjusted to the rate of brine feed to the anode compartments.
Non-monovalent metallic cations in the water fed to the cathode compartments can also lead to the precipitation of insoluble hydroxides in the cathode compartments. The concentration of such cations should preferably also be controlled, typically by softening with cation exchangers. The exchangers must be regenerated and the spent regenerant constitutes a pollution problem.
It has been mentioned above that the cation selective membranes in chlor-alkali cells permit the passage of some hydroxide anions from the cathode compartment to the anode compartment. In fact from about 5 to as much as 40 percent of the electric current can be carried by such hydroxide anions depending both upon the characteristics of the membrane and the concentration of hydroxide in the cathode compartments. Such hydroxide entering the anode compartments can result in a decrease of chlorine production efficiency at the anode through the increase in the direct evolution of oxygen at the anode and the reaction of chlorine with hydroxide to form hypochlorite and chlorate. If the liquid effluent from the anode compartments is resaturated with chloride and recycled to the process, then such chlorate may build up and limit the solubility of chloride in the influent to the anode compartments. Chlorate has been typically controlled by sending to waste part of the effluent from the anode compartments and recycling the remainder. Such "bleeding" to waste constitutes a pollution problem and a loss of chloride. On the other hand the hypochlorite and chlorine content of that portion which is recycled poses a corrosion problem for the chloride resaturation apparatus and the subsequent brine treatment equipment. Adding acid to the anolyte compartment neutralizes some of the hydroxide anions and decreases the build up of chlorate and the evolution of oxygen. Such procedure has been described in U.S. Pat. No. 3,948,737, Cook, Jr. et al and elsewhere.
Finally, most commercially available chloride salts contain appreciable quantities of sulfate. If the effluent from the anode compartments is recycled and resaturated then sulfate may build up in the influent to the anode compartments decreasing chloride solubility and increasing oxygen evolution at the anodes. It is conventional in non-membrane electrolytic chlor-alkali processes in which brine is recycled to control sulfate by adding soluble barium salts thereby precipitating insoluble barium sulfate. However at the extreme alkalinities existing in the interior of cation selective membranes in chlor-alkali cells, the residual soluble barium may precipitate in the interior of the membrane, resulting in increase in energy consumption and eventually in possible destruction of the physical integrity of the membrane as discussed above. Typically therefore the sulfate content in the influent to the anode compartments in recycle systems has been controlled by bleeding to waste a portion of the effluent from the anode compartments and resaturating and recycling the remainder.
Thus the typical membrane chlor-alkali plant has a number of liquid and gaseous wastes which may constitute pollution and economic problems; and requires a number of chemical feeds to prevent increases in the energy consumption per unit of available chlorine produced.
The present invention comprises an improvement over the above discussed prior art techniques particularly as applied to large volume production membrane chlor-alkali cell apparatus where conservation of energy and utilization of process products and raw materials are important considerations in the economic feasibility of such plants. In the method of this invention, this is accomplished by arranging the process steps in a membrane chlor-alkali plant in particular preferred arrangements with respect to each other and by using as process streams various recycle streams. In one preferred embodiment there are no liquid or gaseous wastes from the membrane chlor-alkali plant and simultaneously no increase in energy consumption per unit of chlorine produced from the build up of precipitates in the membranes or from the build up of chlorate and/or sulfate in the recirculated effluent from the anode compartments.